Cooling system

Overview

The Ninja 250 has a basic, fluid-based cooling system, composed of a water pump, a radiator, a thermostat, an electric fan, and an overflow tank. There are also tubes which interconnect the various pieces. Some are built into the motor as coolant passages, and others are external hoses of metal or rubber. For a good overview of how an engine cooling system works, please see this article at howstuffworks.com. A Ninja 250's cooling system is conceptually identical to a car system, it's just smaller to match the smaller engine.

In normal operation, assuming a system in correct working order and filled with coolant, the engine will remain at the correct operating temperature almost entirely through natural convection, as air moves past the radiator. The air moving past the radiator carries away heat, removing it from the engine. In some circumstances, the cooling system either doesn't receive enough air, or doesn't receive cool enough air, and it will engage the electric fan. This typically happens when the temperature is over about 80° F (27° C), or when the bike is stopped or moving very slowly.

The "correct operating temperature" is any temperature between the second low mark and the bottom of the red zone, which are connected together by an arc line. If the gauge shows colder than the second low mark after the engine has been warmed up, or into the red at any time, that indicates a problem with the cooling system. Additionally, the bike's temperature will tend to run hotter during stop-and-go traffic and cooler out on the open road, due to the cooling effect of the wind.

The fan engages at a predetermined point on the temperature gauge (they're not actually tied together in the sense of using the same temperature sensor, though). Many bikes engage the fan just below the red zone, while others have been reported engaging the fan at the mid-way point on the temperature gauge. The most important thing here is that it's entirely normal for your bike's temperature gauge to run all the way up to the bottom of the red zone before the fan comes on.

For more information on troubleshooting the cooling system, please read this article.

A useful analogy

To properly understand how heat and cooling works in the engine, think of the engine as a water tank, and heat as the water. In this analogy, the radiator and cooling system are the drain pump, getting rid of excess water. The more air you've got flowing over the radiator, the more drain pump power you've got.

As you tootle along at a sedate pace on a residential street, your engine is generating a small amount of heat -- pouring water into the tank. The drain pump is more than large enough to get rid of any excess water. As you go faster, the engine generates more heat, filling the tank faster. The drain pump is still up to it, including absolute top-speed on a hot day. However, the tank gets fuller the faster you go, storing more heat energy in the engine. The cooling system can still get rid of enough heat to keep the engine in the right range, but that's about it. The tank fills to a good level, but doesn't overflow.

Now, imagine you get off the freeway and pull up to a stoplight. Your heat/water tank is full, because you've been generating a lot of heat driving fast. The cooling system was able to drain off enough heat to keep things under control as long as you were moving, but the tank filled up in the mean time. Now that you're stopped, the air flowing over the radiator is reduced to zero (which means your drain pump is down to zero pumping power). Suddenly, any excess heat at all (such as running the engine at idle) is enough to overflow the tank, because it's so full. The temperature gauge shoots up until the fan kicks in -- there's the drain pump stepping in at the last minute before the tank overflows. This situation (before the fan kicks in) is shown in the third diagram.